ISSN 0024-4902, Lithology and Mineral Resources, 2019, Vol. 54, No. 5, pp. 374–389. © Pleiades Publishing, Inc., 2019. Russian Text © The Author(s), 2019, published in Litologiya i Poleznye Iskopaemye, 2019, No. 5.

Tectonic-Sedimentary System of the Atlantis‒Meteor Seamounts (North Atlantic): Volcanism and Sedimentation in the Late Miocene‒Pliocene and Position in the Atlantic‒Arctic Rift System N. P. Chamova, *, I. E. Stukalovaa, S. Yu. Sokolova, A. A. Peivea, N. V. Gor’kovaa, A. A. Razumovskiia, M. E. Bylinskayaa, and L. A. Golovinaa aGeological Institute, Russian Academy of Sciences, Pyzhevskii per., 7, Moscow, 119017 Russia *e-mail: [email protected] Received February 19, 2019; revised February 19, 2019; accepted March 13, 2019

Abstract—The paper analyzes original data obtained on the Atlantis‒Meteor seamount system during Cruise 33 of the R/V Akademik Nikolai Strakhov in the eastern North Atlantic. This system is a volcanic rise formed on the Canary abyssal plate and represents one of the key objects for understanding the geological history of opening of the central segment of the . , tephrites, and organogenic terrigenous lagoonal marine sediments dredged from the Atlantis, Plato, and Cruiser seamounts are considered. Petrog- raphy and compositions of the Atlantis and Cruiser basalts reflect significant differences in settings of their eruptions. Well-crystallized vesicle-free olivine basalts from the Atlantis Seamount were ejected under deep- water conditions. Glassy vesicular basalts of the Cruiser Seamount are typical of shallow subaerial eruptions. Evidence for the accumulation of tuff breccias and tuff gravelstones of the Plato Seamount in subaerial set- tings are obtained. Tendencies were revealed in the lithogenetic transformations of organogenic‒terrigenous sediments of the Cruiser Seamount, which were subjected to the high-temperature impact of subaerial lava flows. During the volcanosedimentary lithogenesis, the plant lignite-like matter lost its primary structure and was transformed into anisotropic coke with the wide development of fusinite and pyrofusinite. The studied volcanic occurrences are thought to be related to the final (Late Miocene‒Pliocene) volcanic stage in the seamount system, which predated the destruction of the system, its prograde subsidence, and transformation of islands into guyots.

Keywords: North Atlantic, seamount, tectonics, sedimentation DOI: 10.1134/S0024490219050043

INTRODUCTION seamounts are contradictory. The existing models can Geological Position and Characteristics be divided into two principle groups: hot spots and of the Seamount System structural control. According to the first concept, the systems of the The tectonic-sedimentary system of the Atlan- Corner and Atlantis‒Meteor seamounts in the west tis‒Meteor seamounts is a regional crustal element, and east of the North Atlantic, respectively, mark the which represents a single volcanic rise formed on the subsequent passage of the , Canary abyssal plate and is one of the key objects for Mid-Atlantic Ridge, and over the New understanding the geological history of the North England in the Late Cretaceous (Tucholke Atlantic opening (Fig. 1). and Smoot, 1990). However, this concept cannot offer The volcanic rise is situated about 1500 km to the a satisfactory explanation of the resumption of volca- northwest of Africa, marked by 3750-m isobath, has a nism on some seamounts 20‒30 Ma after their initial length no less than 1000 km, and area over 350000 km2. passage over the hotspot. Inferred plate rollback or It comprises seven large flat-topped seamounts pres- possible relation of the hot spot with the previously ently located at depths from 240 to 800 m. The sublat- formed volcanic edifices through some intralitho- itudinal Oceanographer, Hayes, and Atlantis fracture spheric channels a few kilometers thick is not geologi- zones intersecting the Mid-Atlantic Ridge are termi- cally substantiated. nated in the area of the Atlantis, Cruiser, and Great Models of this group also ignore the fact that the Meteor seamounts, respectively. Atlantis‒Meteor seamount system has a complex In spite of the comprehensive investigations since morphology, which cannot be explained in the frame- 1970s, views on the evolution of the Atlantis‒Meteor work of a single system of geodynamic strains. The

374 TECTONIC-SEDIMENTARY SYSTEM OF THE ATLANTIS‒METEOR SEAMOUNTS 375

Symbols N VolcanicVolcanic rocksrocks tuffstones,tuffstones, Dredge no. limestoneslimestones Route of Cruise 33 Mt.Mt. AtlantisAtlantis FragmentsFragments ofof Bottom topography, m FeFe−MnMn crustscrusts −10388 − −8000 Mt.Mt. TyroTyro −7999 − −7000 34° −6999 − −6000 −5999 − −5750 −5749 − −5500 −5499 − −5250 Mt. Plato −5249 − −5000 −4999 − −4750 −4749 − −4500 −4499 − −4250 Tuffstones,Tuffstones, −4249 − −4000 limestoneslimestones Mt.Mt. CruiserCruiser −3999 − −3750 −3749 − −3500 −3499 − −3250 −3249 − −3000 Mt.Mt. IrvingIrving −2999 − −2750 FragmentsFragments ofof − − − 32° 2749 2500 FeFe−MnMn crustcrust andand −2499 − −2250 fusedfused rockrock −2249 − −2000 fragmentsfragments −1999 − −1750 − − − Mt.Mt. HyeresHyeres 1749 1500 −1499 − −1250 −1249 − −1000 −999 − −750 Mt.Mt. SmallSmall HHyeresyeres −749 − −500 −499 − −450 −449 − −400 −399 − −350 −349 − −300 AtlantisAtlantis FaultFault − − − ° 299 250 30 − − − Mt.Mt. GreatGreat MMeteoreteor 249 200 −199 − −150 Mt.Mt. SmallSmall MMeteoreteor −149 − −100 −99 − −50 MMt.t. ClossCloss 03570 140 210 280 km −49 − −25 −24 − 0 30° 28° W 26°

Fig. 1. Scheme of the performance of geological‒geophysical works during Cruise 33 of the R/V Akademik Nikolai Strakhov in the Atlantis‒Great Meteor area. Compiled using GEBCO data of 30″ Bathymetry Grid. Version 2014-11-03. 2014 (http://www.gebco.net). Red line is the cruise track. Rectangles are the position of dredging stations. Inset shows the position of the studied area in the Atlantic. position of seamounts (Fig. 1) indicates the existence between the spreading ridge and the plume of at least four structural axes: northwestern (Atlantis (Gente et al., 2003; Ribeiro, 2017). Plateau, Cruiser‒Tyro, Great Meteor‒Hyeres); Geologists from the Geological Institute of the northeastern (Plato‒Tyro–Unnamed, Hyeres– Russian Academy of Sciences studied the Atlantis– Irving–Cruiser); latitudinal (Plato); and the least pro- Great Meteor seamount system during Cruises 1 nounced sublongitudinal (Great Meteor‒Hyeres). (1985) (Bebeshev et al., 1984) and 33 (2016) of the R/V Akademik Nikolai Strakhov. This paper reports Models of the second group are based on the kine- the results of the macro- and microscopic study of the matic mechanism of the formation of the seamount volcanic and terrigenous‒humic rocks dredged from system and on the structural control of “hot” anoma- seamount slopes during 2016 expedition (Fig. 1). New lies detected by seismic tomography data (Bebeshev et materials supplement data on the nature of volcanic al., 1989; Georgen, 2011). Similar models relate the rocks obtained during Cruise 1 and highlight previ- evolution of the region with large-scale interaction ously unknown features of the volcanosedimentary

LITHOLOGY AND MINERAL RESOURCES Vol. 54 No. 5 2019 376 CHAMOV et al.

Table 1. Station, coordinates, and depths of the beginning and end of dredging Station no. Initial latitude Initial longitude Final latitude Final longitude Initial depth, m Final depth, m S3302 34°36.4′ ‒30°33.7′ 34°36.0′ ‒30°34.0′ 840 800 S3307 33°11.5′ ‒29°49.0′ 33°12.4′ ‒29°42.2′ 1500 1260 S3312 32°23.0′ ‒27°30.0′ 32°23.5′ ‒27°30.6′ 1800 1670 lithogenesis of terrigenous‒humic sediments under warm-water relatively shallow assemblage: Reticu- the impact of high-temperature subaerial lava flows. lofenestera pseudoumbilicus, Sphenolithus sp., Scyphos- phaera sp.sp. (3 or 4 species), Discoaster surculus, D. brouweri, D. aff. berggrenii, D. prepentaradiatus, Timing of the Seamount Formation Amaurolithus aff. primus (1 specimen.), Ceratolithus Based on the interpretation of magnetic anomalies rugosus (1 specimen), and others. All discoasteraceae (Verhoef, 1984), ocean floor beneath the seamounts are excessively overgrown by calcite are poorly identified; dated at 82‒86 Ma for the Great Meteor, 75‒83 Ma for given this fact, host rocks could be dated by the second Hyeres, 69‒78 Ma for Irving, 68‒77 Ma for Cruiser, half of the Late Miocene (Golovina et al., 1984). 49‒60 Ma for Plateau, 50‒56 Ma for Tyro, and 37‒48 Ma Thus, the age of the organogenic rocks from the for Atlantis. Atlantis slopes spans a stratigraphic interval from Oli- The formation of volcanic buildups began no ear- gocene to Quarter. Sedimentation occurred in a warm lier than Oligocene or early Miocene (Willians et al., relatively shallow setting and was accompanied by the 1983). Based on the K-Ar dating, the age of dredged redeposition of products of the simultaneous volcanic basalts is 33.4 ± 0.5 Ma for Plato and 31.6 ± 0.4 Ma for activity (Bebeshev et al., 1989). Small Hyeres (Ribeiro et al., 2017). The age of the The Cruiser Seamount is characterized by the wide Cruiser seamounts calculated using empirical curves development of diverse organogenic limestones. The of the subsidence of the Atlantic Ocean crust is about limestones from the western slope at a depth of 16 Ma (Tucholke and Smoot, 1990). The minimum 1800‒1300 m yielded nannoplakton assemblage with age for the Cruiser‒Hyeres seamount complex is esti- Coccolithus pelagicus, Discoaster deflandrei, D. exilis, mated to be 13.3 Ma (Duin, 1984). Based on the K-Ar Calcidiscus macintyrei, D. aff. kugleri, D. aff. bellus, and dating of basalts from depths of 600 and 1000 m, the other forms of the lower Middle Miocene (15‒11 Ma)— minimum age of the Great Meteor seamount is 16.3 ± Discoaster exilis zone (Golovina et al., 1984). 0.4 and 10.7 ± 0.5 Ma, respectively (Wendt et al., 1976). The Great Meteor seamount is overlain by the cover (150‒600 m) of post-Middle Miocene (<11 Ma) car- Their Miocene age also follows from the microfau- bonate and pyroclastic rocks and sands (Bebeshev nal analysis of samples dredged from seamount slopes et al., 1989). According to studies (Mironov and Kry- during Cruise 1 of the R/V Akademik Nikolai Strakhov. lova, 2006), the summit surface during the Late Mio- All sedimentary rocks dredged from the slopes of the cene‒Pliocene (11‒2 Ma) was subjected to marine volcanic rises are enriched in the organogenic carbon- abrasion and then subsided to a depth about 300 m, ate. Most of the rocks are organogenic limestones which terminated the reef formation. consisting of remains of foraminiferal shells, organo- genic carbonate detritus, and nannoplankton (Golovina et al., 1984). RESULTS The Atlantis seamount was dredged at different General Characteristics of Dredged Rocks slopes and depths. The lower part of its slope (depth up to 2340 m) yielded detrital limestones consisting of During 2016 expedition, the stony material was foraminiferal shells and nannoplankton embedded in dredged from the slopes of the Atlantis, Plato, and the fine-clastic calcite cement. Nannoplankton is rep- Cruiser seamounts. Dredging parameters are given in resented by the following species: Coccolithus pelagicus Table 1. (large miocenica forms), C. aff. macintyrei, and Dis- Station S3302 on the northeastern slope of Atlantis coaster deflandrei (extreme overgrowing by secondary yielded volcanic rocks, tuffstones, and limestones CaCO3). Based on these forms, the age of the lime- (Fig. 2a). stones was determined approximately as Miocene The basalts are greenish gray and dense. On the (23‒5 Ma). Among the dredged samples are also for- saw-cut section, separate crystals of olivine or idding- aminiferal‒detrital limestones with nannoplankton of site pseudomorphs after olivine are observed in a Quaternary age (Calcidiscus macintyrei and finest homogenous groundmass. It is noteworthy that prac- Gephyrocapsa sp.). tically unaltered and completely replaced crystals Samples collected from the middle part of the occur in a close vicinity to each other in a dense Atlantis slope (depth about 2000 m) contain diverse homogenous matrix. The crystals have both equant

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(a) (b)

1 cm 500 m (c) (d)

1 cm 1 cm (e) (f)

1 cm 1 cm

Fig. 2. Dredged rocks. (a, b) Atlantis seamount, station S3302: (a) contact between and carbonate rocks, (b) mixture of volcaniclastics and bottom sediments, polished thin section, crossed nicols; (c, d) Plato seamount, station 3307: (c) tuff breccia, (d) tuff gravelstone; (e, f) Cruiser seamount, station S3312: (e) outer side of sample S3312-3 (variegated glassy rock with porous structure), (f) polished section of sample S3312-3 (greenish gray glassy fringe with embayments around dark lignite-like material). and elongated shape. Equant olivines reach 0.5‒0.7 cm diversity (Fig. 2b). Planktonic foraminifers are repre- in size. sented by Orbulina universa, Globorotalia menardii, G. crassaformis, G. tumida, G. scitula, G. puncticulata, Chill margins and intense ferrugination as tongues and Globigerina bulloides, which are typical of the penetrating in a carbonate mass at the contact between Miocene‒Pliocene. basalts and limestones indicate the hot contact between lava and sediment in the past (Fig. 2a). Car- Station S3307 from the southern slope of Plato bonate sediments at the contact with basalts are satu- seamount yielded volcanomictic, locally brecciated rated in microfaunal remains of a wide taxonomic reddish brown rocks of variable density (Fig. 2c). The

LITHOLOGY AND MINERAL RESOURCES Vol. 54 No. 5 2019 378 CHAMOV et al. main part of the rocks is made up of irregularly shaped magma eruption. Traces of magmatic flow are randomly oriented fragments varying in size from 1 to expressed by the deflection of microlites around phe- 5 cm. The fragments have massive structure and fine nocrysts. In addition, basaltic eruption on seafloor is gravelly texture. The grains are volcanomictic and confirmed by the entrapment of mud material saturated weakly rounded, while the cement is tuffogenic and in microfaunal remains in a lava flow (Figs. 3a, 3b). Sig- basal. Within brecciation zones, the fragments acquire nificant temperature difference at the lava‒sediment angular shape, while their size varies from 1 cm to a few contact follows from the clear chill margins around the millimeters. The light cement has a calcareous composi- entrapped material. Basaltic eruptions at great depth tion and intensely liberates CO2 when treated with HCl. of Mt. Atlantis are determined from much wider diver- The absence of microfaunal remains in the cement sug- sity of the entrapped microfaunal assemblage as com- gests a chemogenic nature of the carbonate. pared to the reduced subaerial assemblage of Cruiser. Rocks of similar composition but different facies In Cruiser samples (station S3312), glassy rocks are represented by the weakly cemented fine pebbly under microscope reveal the characteristic features of gravelstones, which breakdown manually and bear lava flows. The groundmass has a typical hyalopilitic signs of viscous rock flow (Fig. 2d). Mixing of inco- (felty) texture with plagioclase microlites embedded in herent mass in the past is emphasized by irregularly a glassy matrix. The rock has an amygdaloidal struc- shaped sandy‒carbonate interbeds up to 1 cm thick. ture typical of shallow subaerial eruptions, which pro- The fragments show a good gradational sorting (0.2– duce relatively thin clearly vesicular hyalobasaltic 0.4 cm) and are poorly rounded. The cement is volca- flows. Numerous amygdules are filled with thin pla- nogenic‒sandy fine-grained, and equigranular. gioclase microlites, more rarely, with ferruginous Station S3312 on the eastern slope of the Cruiser aggregate containing pyroxene microcrystals (Fig. 3c). seamount yielded ferromanganese crusts and irregu- Abundant ore minerals are frequently developed along larly shaped nodules of fused porous rocks (Fig. 2e). It amygdule periphery (Figs. 3c, 3f). The amygdules vary is seen in the saw-cut polished sections that they are from rounded to droplike and completely shapeless. mainly made up of the dark rock resembling wood The droplike amygdule filled with palagonitized glass coal, which is enveloped in a greenish gray glassy coat (Fig. 3d) contains clear traces of viscous‒ductile flow (Fig. 2f). The glassy coat strongly varies in thickness with tension cracks typical of pahoehoe lavas. Macro- from a few millimeters to 1 cm and penetrates into the scopically observed penetration of glassy lava in the predominant rock along interlayers and “embay- lignitic mass and flow structures are also observed ments.” It is clearly seen that the glassy fringe is also under microscope (Fig. 3e). Chill margins are weakly developed on the internal surface of elongated cavities developed, although they are observed on separate in the coalified material. Physical properties of the contact areas and around some detrital grains. Lava observed varieties are sharply different: the glassy rock mobility is also supported by the inclusions of host is hard and leaves no mark, whereas the dark rock is rocks. A relatively large foraminiferal shell seen in the easily crabbed by nail or knife and leaves a black mark polished thin section (Fig. 3f), judging from the on a streak plate. absence of pores and arrangement of chambers, can be approximately determined as the representative of Below we present the results of microscopic study Miliolida. This is benthic group inhabiting relatively of the dredged rocks. shallow depths. The finest calcareous interbed in sam- ple S3312-3 contains single coccolithic species Calcid- Basalts iscus leptoporus (Murray and Blackman 1898; Loe- blich and Tappan, 1978), the stratigraphic range of Basalts of the Atlantis seamount (station S3302) which is determined from the base of the Acvitanian have a well-crystallized microlitic texture and are (19.00‒22.82 Ma) to the present time. practically devoid of vesicles, indicating that the melt crystallized at relatively high pressures (Fig. 3a). Phe- nocrysts account for about 10‒15%. They are mainly Tuff Breccia represented by large (usually 0.5‒3 mm, up to 5 mm) olivine crystals of both octahedral and elongated pris- In samples from Mt. Plato (station S3307), we matic or elongated drop-shaped (quench crystals) studied polished thin sections of massive rock varieties shape. Olivine is locally replaced by iddingsite, which that compose tuff breccia (Fig. 4). sometimes forms complete pseudomorphs. Single The fragments are represented by the angular fine clinopyroxene phenocrysts reach 5 mm. The ground- to medium-grained basalts with diverse texture, or, mass is weakly crystallized. It is made up of small more rarely, by altered minerals. Brown tuffa- (about 0.1 mm) plagioclase laths, olivine microlites ceous‒clay matrix is strongly ferruginated. Ore min- (0.25 mm), and vitrified glass. Small rounded ore erals are widespread both in the matrix and in basaltic minerals (0.05‒0.1 mm) compose up to 30% matrix. fragments. Sometimes, segregations of ore minerals The presence of sharply elongated olivine (quench are confined to lithophysis‒cavities formed by air crystals) indicates its rapid crystallization during bubbles (Fig. 4d).

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(a) (b)

500 μm 1000 μm (c) (d)

1 mm 1 mm (e) (f)

0.5 mm 0.05 mm

Fig. 3. Lava flows of the Cruiser and Atlantis seamounts. (a, b) Atlantis, station S3302: (а) microlitic olivine basalt, crossed nicols, (b) microfauna-bearing bottom sediments entrapped by lava flow (fragment of polished thin section outlined by yellow frame in Fig. 3a, parallel nicols); (c‒f) Cruiser seamount, station S3312: (c) glassy amygdaloidal basalt, crossed nicols, (d) flow textures in a deformed vesicle, parallel nicols, (e) contact of lava flow with organogenic‒sedimentary rocks, crossed nicols, (f) microfau- nal remains in the irregularly shaped amygdule, crossed nicols.

Directive textures, flow traces, sorting, as well as Terrigenous‒Humic Sediments microfauna inclusions are absent. Based on the obser- vations, the tuffaceous material was formed from a Station S3312, Mt. Cruiser. Under optical micro- pyroclastic flow erupted in subaerial conditions. scope, the groundmass composing the central parts of the considered nodules has a clearly stratified texture According to (Ribeiro et al., 2017), tephrites were formed by the alternation of plant humic and pelitic also dredged from the northern slope of Mt. Cruiser. terrigenous materials (Fig. 5a).

LITHOLOGY AND MINERAL RESOURCES Vol. 54 No. 5 2019 380 CHAMOV et al.

(a) (b)

1000 m 1000 m (c) (d)

1000 m 500 m

Fig. 4. Tuffogenic sediments of the Plato seamount, station S3307.

The terrigenous grains are poorly and mildly change of mass, compaction, and reduction of volume rounded, and the largest grains reach 0.5 mm across. (shrinkage). The thickness of terrigenous interbeds varies from 0.01 The influence of high-temperature lavas and solu- to 1.0 mm. tions on the transformation (lithogenesis) of humic Both terrigenous and humic interbeds usually have matter from station S3312 of Mt. Cruiser was studied a lenticular shape. They are frequently disrupted by under high magnification with optical and scanning strike with shear displacement of fragments. The dis- microscopes. It is seen in Fig. 6a that - ruption of stratification is also expressed in the forma- genic‒hydrothermal rock saturates the laminated tion of pinch zones, where interlayers are deformed humic matter. Interlayers are split by the intruding and transformed into melange (Fig. 5b). The terrige- material. Organogenic matter acquires porous texture, nous interlayers contain microfaunal remains: cysts of transforming into a natural coke. A change of primary calcic dinoplagellates, benthic foraminifers, and calcic structure of vitrinite microcomponents facilitates the algae (Figs. 5c, 5d). formation of pores and cavities, which imparts the In spite of the general structural similarity with lig- pumice-type appearance (Fig. 6b). The groundmass nites, the plant humus in the transmitted light has retains the relicts of plant matter. a black color, which is not typical of lignites. Lignites Bands of structural vitrinite with clear cellular usually have brown or light brown color. Obvious structure split by vertical cracks are seen under small humification of humic organic matter indicates its magnifications (about 100 times) (Fig. 6c). Thermal high-temperature transformation. Early studies impact caused the transformation of vitrinite micro- showed that even short-term thermal impact of component into another microcomponent, micrinite erupted volcanic material on the organic matter of ter- (agglomerate of organic and mineral matter). The restrial vegetation leads to the sharp and uneven organic matter contains large and small fragments of changes of its composition and properties (Petrova fusinite. Under large magnifications (200‒500 times) et al., 1998; Stukalova et al., 2004). An increase of and, especially, in oil immersion, micrinite bands are temperature stimulates the condensation and ordering converted into a mineral mass saturated with the fine of carbonaceous structure, which is accompanied by a and finest particles of gray vitrinite and fusinite, which

LITHOLOGY AND MINERAL RESOURCES Vol. 54 No. 5 2019 TECTONIC-SEDIMENTARY SYSTEM OF THE ATLANTIS‒METEOR SEAMOUNTS 381

(a) (b)

0.2 mm 0.5 mm (c) (d)

0.1 mm 0.05 mm

Fig. 5. Terrigenous‒humic sediments of the Cruiser seamount, station S3312. (а) Humic material opaque in the transmitted light with terrigenous interbeds; (b) disruption of stratification in the laminae pinch zone; (c) single calcium cyst of Dinoflagellata in a terrigenous interbed; (d) calcite algae in terrigenous interbed. is distinguished by angular shape and bright white expressed in the bed disruptions (Figs. 7a, 7c), boudi- color. Fusinite is transformed into pyrofusinite, which nage, and brecciation on a different scale (Figs. 7b, 7d), shows clear strong mosaic anisotropy (Stach et al., formation of convolute structures (Fig. 7b), and 1978). Vitrinite is destroyed into small fragments and appearance of sandy dikes (Fig. 7a). is replaced by the pyroclastic material. The value of refraction index Ro in the separate microcomponents of vitrinite group increases significantly and becomes Chemical Composition of Terrigenous‒Humic Rocks more than 3%. It is practically impossible to measure The contents and spectra of elements in the terrig- the value of refraction of micrinite microcomponent enous‒humic sediments were determined on a Cam- owing to the presence of the finest organic particles, Scan MV2300 scanning electron microscope coupled which look like spanglets in the volcanic material. with INCA200 for the energy-dispersive X-ray micro- Figure 6d shows a clear cellular structure of plant analysis. tissues: dark fields in the images are the mineral volca- A significant increase of carbon content was estab- nogenic hydrothermal matter, which saturated the lished in terrigenous varieties. In recalculation for plant remains; gray fields are the micrinite microcom- oxides, the CO2 content in the externally terrigenous ponent; and bright white fields are stellate segregations varieties (“light”) is strongly overestimated, exceeding of fusinite and pyrofusinite. The typically lignite in some cases 50% (Table 2). structure of the plant tissues indicates a terrestrial ori- In the dark (humic) varieties, the carbon content gin of the fragments of humic material and makes it reaches 95‒100%, with sharply subordinate amounts possible to relate it with the higher plants (trees and of N, Al, Si, and O (Table 3). the obtained values con- shrubs). firm the concept (based on microstructural study) that Electron microscopic study revealed numerous the rock is natural coke. deformation structures in the organogenic‒terrige- Following assumptions can be made from the X-ray nous thin-laminated sediments (Fig. 7). They are microanalysis with allowance for the laminated tex-

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(a) (b)

(c) (d)

Fig. 6. Types of the transformation of humic material from the Cruise Seamount, station S3312. (а) Welding and shrinkage of the humic matter during injection of the volcanogenic material; polished section, reflected light, magn. 100; (b) porous texture of micrinite, reflected light, immersion, magn. 100; (c) vitrinite bands, micrinite, fragments of fusinite, reflected light, immersion, magn. 100; (d) clear cellular texture of plant tissues: dark fields show the saturation of plant remains in the mineral volcano- genic‒hydrothermal material; gray fields are microcomponent micrinite; bright white fields are stellate segregations of fusinite and pyrofusinite, polished section, reflected light, oil immersion, magn. 200.

ture of the rocks. The thermal impact on lignite matter CO2-saturated fluids saturated the laminas with the made up mainly of the higher plant fragments leads to terrigenous and likely sapropel material, thus facilitat- the redistribution of components. Thermal reworking ing the leaching and redeposition of some compo- caused a removal of volatiles and mobile components nents. The removal of Fe, Ca, Na, Si and, to lesser from the lignite organogenic material and its transfor- extent, K was accompanied by the enrichment in CO2 mation into the highly carbonaceous coke. Mobile (Table 2).

Table 2. Chemical composition of light interbeds in the terrigenous‒humic sediments, wt % t Sample SiO2 Al2O3 FeO MgO CaO Na2OK2OCO2 Total S3312/t14 46.67 10.17 0 1.06 0 3.84 3.17 35.09 100.00 S3312/t17 63.84 0.73 0 0 0 0 0 35.43 100.00 S3312/t21 35.52 8.76 0 0.74 0 1.19 2.14 51.56 99.91 S3312/t22 30.78 12.24 1.08 0.5 0 0.15 1.95 53.3 100.00 S3312/t25 47.95 10.33 0 0.95 0 0 3.28 37.5 100.01 S3312/t26 32.53 14.73 0 0.53 0 0 1.89 50.34 100.02 S3312/t46с 55.78 1.19 0 2.05 4.64 9.04 0.65 26.65 100.00 Analyses were carried out on a SEM CamScan MV2300, gold sputtering. Position of measurement points is shown in Figs. 7а–7c. FeOt as total iron.

LITHOLOGY AND MINERAL RESOURCES Vol. 54 No. 5 2019 TECTONIC-SEDIMENTARY SYSTEM OF THE ATLANTIS‒METEOR SEAMOUNTS 383

(a) (b) t13 t21

t14 t22 23

t25 t26 t16

t18 t28 t17 100 μm 100 μm (c) (d) g1

g5

g2 ore mineral t46b

g9 t46c 200 μm 50 μm

Fig. 7. Results of the electron microscopic study of terrigenous‒humic and volcanic rocks of the Cruiser Seamount, station S3312. (a‒c) Deformation structures in the terrigenous‒humic rock; (d) hyalopilitic structure of basalt. Indices show the mea- surement points of rock composition on a CamScan MV 2300 scanning microscope coupled with INCA 200 for the EDS X-ray microanalysis. Chemical composition in the measurement points is shown in Tables 2‒4.

This tendency is best expressed in measurements Chemical Composition of Basalts t21, t22, and t26 along the sandy dike that obliquely Basalts of the Atlantis and Cruiser seamounts sig- crosscuts the subhorizontal stratified terrige- nificantly differ from each other in the degree of alter- nous‒humic sediments (Fig. 7b). They demonstrate a ation, which affected the chemical compositions and sharp (18‒30%) decrease of SiO2 at proportional choice of the study method. growth of CO2 (Table 2). This observation reflects the Basalts of the Atlantis seamount have a fresh mobility of highly reactive fluids and their selective appearance and can be analyzed by the XRF method confinement to the weakened zones of the rock. (Table 4). The obtained compositions in general are typical of within-plate alkali volcanic rocks. In the Although the presence of glassy coat around the Na2O + K2O vs. SiO2 discriminant diagram (LeBas et terrigenous‒humic rocks seems to be the ground for al., 1986), data points of volcanic rocks are restricted the isochemical conditions of volcanosedimentary to the boundary between basanites and picrite basalts. lithogenesis, the existing data are insufficient to sub- In the Na2O + K2O–MgO–FeOtotal diagram (AFM) stantiate the geochemical closure of the system. At the (Irvine and Baragar, 1971), they are grouped in the same time, significant part of the redistributed min- field of calc-alkaline rocks. The Fe mole fraction (f' = eral material has remained within the system. In par- FeO + Fe2O3 + MgO + TiO2) in lava flows varies ticular, among newly formed minerals are abundant within 22‒25. high-Fe ore minerals, which are identified under elec- Basalts of the Cruiser Seamount were previously tron microscope as small bright white grains dispersed studied during Cruise 1 of the R/V Akademik Nikolai in a stratified mass (Figs. 7a, 7b, 7d). Strakhov. Analysis of the chemical composition

LITHOLOGY AND MINERAL RESOURCES Vol. 54 No. 5 2019 384 CHAMOV et al. showed that all studied volcanic rocks were subjected Table 3. Element composition of dark interbeds in the ter- to significant postmagmatic alterations, which were rigenous‒humic sediments accompanied by intense transformation of their pri- Sample Element wt % at % mary chemical composition (Zolotarev et al., 1984). The main chemical transformations in the rocks are S3312/t13 С 89.88 91.90 related to the significant Si removal, insignificant Mg O 7.04 5.40 loss, and significant increase of the Fe content. In N 3.08 2.70 addition, the rocks have high P2O5 contents (up to 10 wt %), which is not typical of unaltered magmatic S3312/t16 C 92.94 94.69 rocks. Zolotarev et al. attempted to reconstruct the O6.775.18 chemical composition of volcanic rocks by recalculat- Al 0.29 0.13 ing the compositions of altered rocks with subtracting the formula amounts of apatite and calcite. However, S3312/t16 C 77.88 83.71 the comparison of calculated compositions of strongly O17.5314.15 altered rocks with the bulk composition of basalts of Si 3.13 1.44 the Atlantis seamounts does not seem correct. Al 1.46 0.70 Contents and spectra of elements in the basalts from station S3312 were determined using CamScan S3312/t23 C 83.40 86.29 MV2300. The hyalopilitic texture of rocks is best O 9.19 7.13 expressed under electron microscope: plagioclase N 7.42 6.58 microlites are usually separated from each other by S3312/t28 C 92.12 93.96 glass (Fig. 7d). The major element of the microlites is Al (80‒100%), while Si, Fe, Mg, and Ca occur in sub- O 7.88 6.04 ordinate amounts. Analyses were carried out on a SEM CamScan MV2300, Ore minerals observed as bright white grains under with gold sputtering. Position of measurement points is shown in Figs. 7a, 7b. electron microscope form numerous small and single porphyritic crystals in a glassy matrix. Large crystal has the following composition (Fig. 7d) (in wt %): Evidently, erosion accompanied the prograde sub- FeO 71.4, SiO2 5.27, Al2O3 3.1, and SO3 0.23. sidence of the volcanic rise. It is reasonable to suggest Representative measurements of the composition that before this stage, the peaks of volcanic buildups of glassy matrix of the basalts are shown in Table 4. In have emerged above sealevels. Analysis of the modern the Na2O + K2O vs SiO2 discriminant diagram (LeBas morphostructure and depth of the Cruiser seamount et al., 1986), data points of matrix of Mt. Cruiser vol- suggests that initially the seamount have raised canic rocks define a continuous basalt‒basaltic 400‒500 m above sea level (Bebeshev et al., 1984). andesite‒andesite series. In the Na2O + K2O–MgO– The study of terrigenous‒humic sediments points FeOtotal (AFM) diagram (Irvine and Baragar, 1971), to the subaerial sedimentation settings in the past. The their data are grouped in the field of calc-alkaline laminated texture of the organogenic‒terrigenous rocks. The Fe mole fraction (f' = FeO + Fe O + MgO + mass indicates its multiple accumulations in a semi- 2 3 isolated shallow-water basin with periodical influx of TiO2) in lava flows is no more than 16, which corre- sponds to their mesocratic appearance. the terrigenous material and plant fragments. The sed- iments are mainly made up of fragments of the terres- Obviously, direct comparison of the bulk composi- trial higher plants (remains of shrubs and tree tions of the Atlantis basalts and the glassy matrix of the branches), which follows from the clear cellular tex- Cruiser basalts is incorrect. In addition, the low alkali ture of plant tissue. The contribution of shal- content in the Cruiser basalts is likely related to sec- low‒marine plant material (calcareous algae or algal ondary alterations. However, the complex of observa- mats) was insignificant. tions indicates that the calc-alkaline rocks of both vol- Partial isolation of the basin from ocean follows canic buildups belong to the single trachybasalt‒tra- from a poor taxonomic composition of microfauna. chyandesite series. The absence of traces of active water dynamics and the pelite size of terrigenous material serve as evidence in DISCUSSION favor of the quiescent sedimentation setting. Such conditions could exist in a shallow bay (lagoon), Volcanic and Sedimentation Settings in the Miocene which served as a local depocenter for the adjacent The same bathymetric levels at different flanks of volcanic buildups. the seamount system point to a subsynchronous sub- Tuffaceous sediments of the Plato Seamount do sidence of all parts of the volcanic rise. Flat-topped not contain fragments of marine microfauna in the peaks suggest the involvement of the volcanic buildups cement and were likely formed under subaerial condi- in a zone of high hydrodynamic activity. tions. Tuff breccia was formed owing to the compac-

LITHOLOGY AND MINERAL RESOURCES Vol. 54 No. 5 2019 TECTONIC-SEDIMENTARY SYSTEM OF THE ATLANTIS‒METEOR SEAMOUNTS 385

Table 4. Bulk chemical composition of Atlantis basalts and glassy matrix of Cruiser basalts, wt %

Sample SiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Na2OK2OP2O5 L.O.I. Total Atlantis seamount 3302/b1a 39.58 3.99 15.66 7.18 5.87 0.17 7.63 11.66 2.61 0.64 0.70 3.67 99.35 3302/b1b 39.49 4.07 17.47 6.74 5.83 0.18 6.22 11.54 2.85 0.55 0.92 3.50 99.36 3302/b1c 40.94 3.98 15.38 8.14 4.84 0.17 7.46 11.51 2.92 0.84 0.64 2.65 99.46 3302/b3 38.97 4.14 15.29 7.31 6.13 0.19 7.45 12.01 2.88 0.82 0.60 3.54 99.33 Cruiser seamount 3312/g155.441.3025.560.007.870.002.483.070.393.890.000.00100.00 3312/g255.410.8824.310.008.600.003.013.590.613.590.000.00100.00 3312/g5 57.24 1.64 24.95 0.00 5.99 0.00 2.04 4.01 0.56 3.57 0.00 0.00 100.00 3312/g957.950.0022.770.007.970.002.843.830.803.840.000.00100.00 3312/g15 54.75 1.06 24.81 0.00 8.34 0.00 2.66 4.00 0.44 3.94 0.00 0.00 100.00 3312/g16 58.46 1.37 21.64 0.00 6.57 0.00 1.67 4.35 0.69 5.24 0.00 0.00 99.99 3312/g17 54.00 1.33 23.47 0.00 8.68 0.00 2.48 4.53 1.54 3.97 0.00 0.00 100.00 3312/g18 56.12 1.60 23.27 0.00 7.38 0.00 2.71 4.25 0.45 4.22 0.00 0.00 100.00 3312/g19 55.55 0.96 22,52 0.00 7.88 0.00 2.67 4.27 0.42 4.30 1.43 0.00 100.00 3312/g20 56.02 1.87 22.90 0.00 8.38 0.00 2.07 3.49 0.70 3.71 0.87 0.00 100.01 3312/g21 54.97 1.18 22.86 0.00 8.61 0.00 2.66 4.39 0.63 3.68 1.04 0.00 100.02 3312/g22 52.13 1.28 20.97 0.00 8.06 0.00 1.86 4.42 0.48 3.96 2.24 4.61 100.01 3312/g23 55.17 0.57 23.82 0.00 8.14 0.00 2.88 4.36 0.84 3.92 0.29 0.00 99.99 3312/g24 55.20 1.34 23.13 0.00 8.69 0.00 2.40 3.88 0.73 4.05 0.59 0.00 100.01 3312/g25 52.84 1.44 21.48 0.00 8.10 0.00 2.10 3.76 0.49 3.75 1.43 4.60 99.99 3312/g26 53.80 1.53 23.64 0.00 8.71 0.00 2.10 4.00 0.94 3.85 1.43 0.00 100.00 3312/g27 54.45 1.27 24.14 0.00 9.30 0.00 1.76 3.91 0.76 3.69 0.71 0.00 99.99 3312/g28 55.05 0.96 23.47 0.00 9.19 0.00 2.53 4.09 0.64 4.01 0.07 0.00 100.01 3312/g29 54.32 1.23 24.10 0.00 8.14 0.00 2.22 4.33 0.82 4.06 0.79 0.00 100.01 3312/g30 55.16 1.73 23.56 0.00 7.86 0.00 2.06 3.72 0.92 4.09 0.89 0.00 99.99 The chemical composition of basaltic samples S3302/b1a–S3302/b3 was analyzed by the XRF; the chemical composition of glassy matrix of basaltic samples 3312/g1–3312/g30 was determined on a SEM CamScan MV2300, coal sputtering. Position of measurement points 3312/g1–3312/g9 is shown in Fig. 7d. tion and cementation of the nonsorted coarse-clastic Basalts dredged from the slopes of the Atlantis and loose volcaniclastic material in the sand-sized tuff Cruiser seamounts resemble those of basalts of these cement. Facies similarity of tuff gravelstones with the two volcanic stages in terms of structure and composi- alluvial‒proluvial sediments of temporal water tion. However, there are no direct grounds (e.g., streams also serves in support of the subaerial sedi- microfossil assemblage entrapped by lava flows) to mentation settings. exclude their subsynchronous development. Differ- Previous study showed that the Cruiser Seamount ences in the character of volcanism could be caused by was formed in two volcanic stages (Bebeshev et al., local variations in tectonic settings within a volcanic 1984). The first stage was marked by the submarine rise. Basaltic samples dredged from the slopes of the eruption of practically vesicle-free ankaramite– Plato, Small Hyeres, Great Meteor, Small Meteor, hawaiite melts at large depths. At the second stage, and Closs seamounts have both vesicular and porphy- already uplifted volcanic buildup ejected mugearite– ritic structure with variable composition of phe- trachybasaltic lavas, which are characterized by strong nocrysts, while the modal proportions and structural vesicularity, heterogeneous distribution and intricate relations significantly vary between seamounts shape of vesicles, and fluidal structure of the rocks. (Ribeiro, 2017).

LITHOLOGY AND MINERAL RESOURCES Vol. 54 No. 5 2019 386 CHAMOV et al.

Assumptions derived from the available data AARS from the south northward through the pole, but require further verification. In particular, basalts from clear decreasing age sequence is not observed. the Small Hyeres seamount slopes have trachytic An important feature of the AARS is the presence structure and contain minor amount of olivine and of magmatic and amagmatic segments (Lundin et al., pyroxene (Ribeiro, 2017), which is also typical of 2018). This phenomenon is best expressed in the Cruiser basalts. Both sea mounts are extreme buildups youngest Arctic segment, where pre-spreading igne- of the southwestern‒northeastern lineament on the ous provinces were established along the periphery of southern flank of the volcanic rise (Fig. 1). Without surrounding continents. They are traced from the excluding the possible higher maturity of these basalts, Siberian traps through the high Arctic to modern Ice- we may suggest that the style of eruptions depended on land (Gaina et al., 2014). Understanding the general the spatial position of extensional area. tendency is complicated by the fact that igneous prov- The presence of altered humic material on the inces in the same oceanic segments have also been Cruiser seamount indicates in support of the later vol- formed after spreading initiation. They include the canic activity. Its accumulation in the shallow Azores Archipelago and AARS seamounts in its fram- lagoonal‒marine conditions could not precede the ing. This indicates a pulsed influx of the deep-seated formation of volcanic buildup. Moreover, the accu- material. Mechanism of the spreading systems– mulation and maturation of humus required sufficient plumes interaction is likely universal and can be exem- time and appropriate sedimentation setting. plified by seamounts surrounding the Azores. Results of our studies allowed us to link the studied The planetary scale of these processes follows from sediments with the second phase of volcanic activity. the potential number, position, and morphological This follows from the structural-textural features of characteristics of seamounts (Morato et al., 2013). lava flows (glassy and vesicular groundmass and vis- Only within the Oslo–Paris Convention for the Pro- cous-ductile deformations) and significant secondary tection of the Marine Environment of the North-East alterations, including the development of clay miner- Atlantic (OSPAR)-protected territory, 557 large sea- als, zeolites, carbonates, and phosphates in vesicles mounts standing more than 1000 m above sealevel and other cavities up to their pervasive replacement by were mapped in the northeastern Atlantic. This list is palygorskite. not complete, because it is constrained by the tasks of studies and parameters applied to reveal the subma- rine structures. The number of objects will be at least Tectonovolcanic Rises as an Attribute doubled by adding seamounts located along AARS to of the Atlantic‒Arctic Rift System the south and north of the OSPAR-protected territory. Uncertainty with genesis of the tectonosedimenta- Based on the spatial position, the seamounts can be tion system of the Atlantis‒Meteor seamounts is par- subdivided into two principle groups. The first, largest tially related to the fact that the existing models of its group includes seamounts localized in rift valleys or evolution are confined directly to the studied object along their walls, including the along-axial Arctic vol- and are of local application. Even consideration of the canic structures from the Reykjanes to the Gakkel hot spot or analysis of influence of the Azores plume rifts. It is noteworthy that the intensity of their evolu- on the system evolution does not go beyond the scope tion and distribution density on seafloor practically of regional studies. does not depend on the spreading rate. The well- An alternative approach consists in the consider- arranged volcanic buildups and active volcanoes were ation of structural ensemble of the given region as found in the ultraslow spreading ridges such as the a part of numerous volcanic buildups, the develop- Knipovich and Gakkel rifts. ment of which is tightly related to the progressive The second group includes seamounts located at crustal extension in the Atlantic‒Arctic Rift System significant distance from rift valleys and shows no (AARS). This system is a divergent boundary about obvious link to the main tectonic lineaments—the pas- 1800 km long extending from the Bouvet triple point sive parts of transform faults. However, in some cases, up to the Iceland plume, the main plume of the Arctic these seamounts are grouped into chains oriented at zone, and further northward through the Knipovich different angles relative to spreading. Seamounts and Gakkel ridges. The AARS has a frontal junction framing the AARS axis were formed through the inter- with Eurasia near the Lena River mouth. Judging from action of prograding rift system with the pulsed supply seismicity (Avetisov, 1996), the continuation of the of deep-seated melts. The latter could be discharged divergent boundary into the continent has a diffuse either in the rift axis or in its framing, thus forming character and its exact geometry has not been deter- volcanic buildups of the younger age relative to their mined yet. protolith. The spatial distribution of the superimposed The AARS comprises crustal segments with volcanism can be controlled by different factors such spreading age varying from 170 Ma in the central seg- as the fault system of transform or regmatic nature. ment to 54 Ma in the Arctic segment (Sokolov, 2017). This scenario of intraplate magmatism operates in the These differences reflect a general evolution of the basins of all AARS magmatic segments.

LITHOLOGY AND MINERAL RESOURCES Vol. 54 No. 5 2019 TECTONIC-SEDIMENTARY SYSTEM OF THE ATLANTIS‒METEOR SEAMOUNTS 387

One of the main tendencies in the AARS evolution AARS and can be related to the affiliation of the sys- is the formation of paired objects located symmetri- tem to the central segment with the longest evolution cally relative to the rift valley. For instance, the Sierra- and maximum opening. Leone‒Ceara paired rise system in the Equatorial Atlantic forms a symmetrical (bulls eyes) pattern of minima of the anomalous magnetic field (AMF). CONCLUSIONS Such configuration could be formed owing to the The study of rocks dredged from the Atlantis, short-term intersection of a plume branch with the Plato, and Cruiser seamounts allowed us to recon- MAR axis and the short-term pulsed melt influx along struct volcanic and sedimentation settings that have this branch with the subsequent quiescence of mag- existed in this area in the Miocene. matic activity. A displacement of continuing activity of the AARS plume axis results in the Rio Grande‒Wal- Basalts of the Atlantis and Cruiser seamounts were vis ridge type configuration. The present-day active formed in different settings. Well-crystallized vesicle- plumes near the AARS axis could be supplied through free olivine basalts of the Atlantis Seamount resulted several pathways in the upper mantle and form sea- from the deep-water eruptions under high pressure. mount chains in the framing. Analysis of AMF Their eruption at great depths is also supported by the between the Atlantis and Vema faults located, respec- taxonomic composition (typical of open basins) of tively, in the Northern and Southern (Equatorial) microfauna entrapped with the bottom sediments. Atlantic allowed us to reveal six paired zones relative to Well-pronounced chill margins indicate a sharp tem- MAR (Sokolov, 2017). Analysis of small (<50 km) perature contrast on their contact with sediments. The equant negative gravity anomalies within the MAR Cruiser basalts show amygdaloidal structure typical of flanks showed that they correspond to paired land- the shallow subaerial eruptions, which produce rela- forms that are symmetric relative to the rift valley. tively thin and highly vesicular hyalobasaltic flows. In A bright example of such anomalies is a chain consist- addition, the basalts contain inclusions of the terrige- ing of three pairs and one central anomaly immedi- nous‒humic material of subaerial origin. Differences ately south of the Strakhov Fault (4° N). Paired struc- in the conditions of basaltic eruptions could be caused tures of different scale were also established in the Arc- by both local variations in tectonic settings within the tic segment of the AARS, in the young oceanic volcanic rise and by the temporal evolution of melts structures restricted to the Norwegian Greenland and from ankaramite to mugearite. Arctic provinces, which mark the North Atlantic con- The absence of marine microfauna in the cement tinuation. For instance, the local paired landforms are of tuffogenic sediments from the Plato Seamount the Litvin and Pogrebitsky seamounts located, respec- likely indicate that they were formed under subaerial tively, on the western and eastern flanks of the Knipo- conditions. Tuff breccias were formed by the compac- vich rift valley (Chamov et al., 2010). tion and cementation of the unsorted coarse-clastic The geographic position, structure, and composi- loose volcaniclastic material in the sand-sized tuff tion of basalts of the Atlantis‒Meteor tectonosedi- cement. Facies similarity of tuff gravelstones with allu- mentation system are well consistent with the above vial‒proluvial sediments of temporal water streams mentioned models of the formation of within-plate also supports the subaerial sedimentation settings. volcanism on different AARS segments both in the The study of terrigenous‒humic sediments of the plume axis and off-axis, with allowance for the dis- Cruiser Seamount indicates subaerial sedimentation placement of plate systems relative to the deep path- settings in the past. The laminated texture of the ways of mantle material. The Atlantis‒Meteor system organogenic‒terrigenous mass points to its multistage is the eastern part of the paired structure ensemble that accumulation in a semi-isolated shallow marine basin is symmetrical relative to the AARS rift valley, the with periodical influx of the terrigenous material and western flank of which composes the Corner sea- plant fragments. The sediments are mainly composed mount group. Both parts of the former ensemble are of fragments of the terrestrial higher plants (remains of located on flanks of the Oceanographer, Hayes, and shrubs, tree branches), which follows from a clear cel- Alantis transform faults, which are spaced from each lular structure of plant tissues. The shallow marine other at about 2000 km and are marked by 34 paired plant material (calcareous algae and algal mats) occurs magnetic anomalies (Roest et al., 1992). in subordinate amount. The fact that the basin was Due to the best structural conjugation of the Cor- partly isolated from ocean is confirmed by the poor ner, Plato, and Cruiser seamount groups along the taxonomic composition of microfauna. The absence Hayes Fault, the latter can be considered as axis of the of traces of active water dynamics and the pelite size of entire transform fault system. Available data allowed terrigenous material point to the quiescent sedimenta- us to relate the initiation of the system and the devel- tion setting. Such conditions could arise in a shallow- opment of these linear extension zones and subse- water bay (lagoon), which served as the local depocen- quent volcanism with the decompression melting of ter for the adjacent volcanic buildups. the underlying mantle beneath them. The rift axis is The study of material dredged from the eastern not displaced along these faults, which is not typical of slope of the Cruiser Seamount indicates that the shal-

LITHOLOGY AND MINERAL RESOURCES Vol. 54 No. 5 2019 388 CHAMOV et al. low-water terrgenous‒humic lagoonal marine sedi- Gaina, C., Medvedev, S., Torsvik, T.H., et al., 4d Arctic: ments were subjected to the high-temperature impact A glimpse into the structure and evolution of the Arctic in of subaerial lava flows. Widespread stellate segrega- the light of new geophysical maps, plate tectonics and to- tions of fusinite and pyrofusinite indicate that the mographic models, Surv. Geophys., 2014, vol. 35, pp. 1095– organic matter acquired anisotropic properties under 1122. the high-temperature impact. Analysis of the chemical https://doi.org/10.1007/s10712-013-9254-y composition of rocks showed that the lignite matter Gente, P., Dyment, J., Maia, M., and Goslin, J., Interac- made up mainly of the higher plant fragments under tion between the Mid-Atlantic Ridge and the Azores hot high temperature lost volatiles and mobile elements spot during the last 85 Myr: Emplacement and rifting of the and was transformed into the highly carbonaceous hot spot-derived plateaus, Geochem. Geophys. Geosyst., 2003, vol. 4, no. 10, pp. 1–23. natural coke. The mobile, CO2-saturated fluids pene- trated the laminas with the terrigenous and likely sap- Georgen, J.E., Lithospheric control on the spatial pattern ropel material and facilitated the leaching and redepo- of Azores hotspot seafloor anomalies: Constraints from a sition of some components. The removal of Fe, Ca, model of plume- interaction, Geophys. Rev. 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